1) Field of the Invention
The present invention relates to an image formation apparatus and a heater control method for performing light-on control of a heater used in a fixing device by PWM-controlling the frequency of an AC power supply of the heater.
2) Description of the Related Art
Conventionally, a halogen heater is used as a heater for an image formation apparatus. This halogen heater has tungsten filament as well as inert gas filled into a glass tube, and the temperature of this heater is controlled by controlling the current-carrying time of the alternating current (AC) power supply by a semiconductor device such as a triac.
Generally, there are known two types of methods for the temperature control. The first is phase control. The triac is turned on after certain time passes from the zero-cross point of the AC power supply. Thereafter, when the polarity of the triac is inverted, the triac is turned off. By controlling the time (phase angle) from the zero-cross point until the triac is turned on, the temperature of the heater is controlled. The second is ON/OFF control. A half period of the AC power supply is set as a minimum ON/OFF unit, and the heater is not turned on or off halfway along the half period.
As an example of the phase control, there is proposed a power supply control device disclosed in Japanese Patent Application Laid-Open No. 2000-321920. The power supply control device restricts the phase angle of alternating current power supplied based on the temperature detected by a temperature detection unit, to a designated phase angle in supplying power to a heating unit or cutting off the power thereto, thereby suppressing the influence of a rush current, a harmonic current, and fluctuations in power supply voltage over the other equipment when the power is turned on or the power is cut off.
The phase control has a possibility of ensuring highly accurate temperature control. However, since an input current does not take a sine wave, it disadvantageously takes the form of a distorted input current waveform that contains a harmonic current.
Meanwhile, the advantage of the ON/OFF control is in that the waveform of an input current is a sine wave when the input current passes and therefore a harmonic current is smaller in quantity. However, the ON/OFF control has a disadvantage of fluctuations in power supply voltage since the minimum unit of ON/OFF is the half period of the AC power supply.
The power for the fixing heater used in a large-sized copier or a high-speed copier tends to be insufficient. However, if the number of options installed into the copier as a system is small or DC load imposed on the operation thereof is light, it is desirable to supply sufficient power to the fixing heater.
Taking the above into consideration, it is also possible to install an ammeter, to set the power capacity of the fixing device higher, and to accurately control the power by the phase control. If so, the phase control is conducted, which causes a problem to occur such that an input current waveform is distorted as explained above in the conventional method of turning on or off the power by the triac.
Accordingly, the power capacity of the fixing heater is determined so as not to exceed the power specified as that of the system even when the heater is fully lit on. The power for the fixing heater inevitably amounts to the power obtained by subtracting a primary-side power at the time of maximum DC load from the power specified as that of the system.
Moreover, there is known that if the AC power supply connected to the fixing heater is pulse-width-modulation (PWM)-controlled with high frequency, the input current averagely flows during the half period and the waveform of the input current becomes a rough approximation of a sine wave, making it possible to solve the problems of both the harmonic wave and the voltage fluctuation.
The present invention has been achieved in order to solve the above problems. It is an object of the present invention to obtain an image formation apparatus and a heater control method capable of preventing the input current waveform of an AC power supply connected to a fixing heater from being distorted, suppressing fluctuations in power supply voltage, and supplying maximum power to the fixing heater.
The image formation apparatus according to one aspect of this invention, comprises a heater used in a fixing device, and a detection unit that detects zero-cross points of a frequency of an alternating current (AC) power supply connected to the heater. The image formation apparatus also comprises a control unit that controls light-on of the heater by pulse-width-modulation (PWM)-controlling the frequency of the AC power supply that controls a duty width of a PWM control signal for the frequency of the AC power supply for a half period that corresponds to a period between the two adjacent zero-cross points detected by the detection unit, and that controls an amplitude of an input current from the AC power supply so that a waveform of the input current becomes a rough approximation of a sine wave.
The heater control method according to another aspect of this invention, comprises the step of detecting zero-cross points of a frequency of an AC power supply connected to a heater used in a fixing device. The heater control method also comprises the steps of controlling light-on of the heater by PWM-controlling the frequency of the AC power supply, controlling a duty width of a PWM control signal for the frequency of the AC power supply for a half period that corresponds to a period between the two adjacent zero-cross points detected at the step of detecting, and controlling an amplitude of an input current from the AC power supply so that a waveform of the input current becomes a rough approximation of a sine wave.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.